Today, not only mobile telephones but also other mobile terminals, such as PDAs (Personal Digital Assistants), notebooks, etc., exchange data with wireless networks via radio interfaces. Typically, a radio base station of a network serves the mobile terminal by routing data received from the terminal through the network towards the recipient, and by transmitting data received from the network side over the radio interface towards the mobile terminal.
The available transmission resources such as frequency (bandwidth), time (timeslots available in transmission frames) and transmission power are generally limited and therefore have to be used as efficiently as possible. In this respect, the base station controls not only the resource parameters for downlink transmissions (from the base station to the terminal), but also for the uplink transmissions (from the terminal to the base station). For the uplink, the base station has to ensure that the mobile terminal is synchronized with the transmission scheme of the radio interface with appropriate accuracy. To this end the base station analyzes a received uplink signal, derives appropriate adjustment values for the uplink transmission parameters used by the terminal and sends synchronization information indicating the necessary adjustments towards the mobile terminal, which then has to adjust its transmission parameters accordingly. The synchronization information may comprise (for one or more of the transmission parameters) either a value for the transmission parameter itself or an adjustment value enabling the terminal to adjust the transmission parameter accordingly.
Whereas the synchronization of the mobile terminal may be performed in a straightforward manner in case of an existing uplink connection during which signals from the terminal are continuously received and analyzed at the base station, no such analysis is possible in case the terminal wants to connect for the first time (for example at power-up or during a handover) or from a standby status (in which the terminal only listens to the downlink). In these circumstances a random access procedure has to be performed to achieve synchronization.
The random access procedure serves several other purposes also. For example the terminal needs an (at least temporary) identification to be addressable by the base station. Further, the terminal needs to provide some information such that the base station can decide to grant access to the network. Additionally, some information regarding the required uplink/downlink resources has to be exchanged.
Any resource allocation, for example the resource allocation for the random access procedure itself, is based on the transmission scheme utilized over the radio interface. A straightforward allocation scheme may comprise reserving in a semi-permanent way a particular channel, which may be specified by particular orthogonal resources such as time slot(s) and frequency band(s), which then may be used by the terminal for its uplink transmission. However, the reserved resources are then blocked for use by other terminals, which means that resources are wasted in case the mobile terminal does not use the reserved channel to its full extent (for example because the transmission buffer is momentarily empty). On the other hand, the reserved resources may not be sufficient to allow fast transmission of data in case the transmission buffer is full. As a further drawback, this straightforward allocation scheme is dimension-limited, i.e. only a particular number of users (determined by the available orthogonal resources) can be served.
During the random access procedure, uplink transmission parameters such as time, frequency and power are not accurately aligned with the transmission scheme as predetermined by the radio base station. Therefore, in an orthogonal scheme additional resources have to be provided for the random access to avoid interference of the typically misaligned random access bursts with synchronized normal bursts, which are, for instance, transmitted in neighbouring time slots. These extra resources comprise for example guard periods and guard bands in the time and frequency dimension, respectively.
In another transmission scheme known for example from WCDMA (Wideband Code Division Multiple Access) systems, the uplink channel provided to the terminal is based on non-orthogonal resources, i.e. the terminal may sent its data using arbitrary time slots and/or frequency bands. While such a system is not dimension-limited, inter-user interference increases with increasing numbers of concurrently sending terminals, which degrades the performance and thus the throughput. In addition, fast power control is required to accurately control the transmission power of each terminal.
There is thus a need for a resource-efficient random access procedure which addresses at least some of the drawbacks set forth above.